November 2, 2005

Revealed: Why London’s Millennium Bridge wobbled

LONDON (Reuters) - A natural phenomenon rather than a
design fault caused London's Millennium Bridge to wobble and
sway, forcing its closure just two days after opening in 2000.

The elegant pedestrian walkway was conceived as a blade of
light linking the south bank of the River Thames to the City of
London.

But as large crowds walked across the steel structure on
opening day in June 2000, the 320-meter long bridge swayed from
side to side because of a phenomenon known as collective
synchronisation.

"The phenomenon was that people who were walking at random,
at their own favorite speed, not organized in any way
spontaneously synchronized," said Steven Strogatz, of Cornell
University in Ithaca, New York.

"That's the phenomenon. Why did they all start moving in
step? They did it unconsciously. That is what nobody had
thought about and engineers did not anticipate."

The applied mathematician and expert on the phenomenon said
collective synchronisation is now something engineers will have
to consider when designing bridges.

He and colleagues at Cornell and other universities in the
United States, Britain and Germany have devised a theory based
on what happened to the Millennium Bridge to estimate how much
damping or stabilization is needed in footbridges.

Their findings are published in the science journal Nature.

"We think our theory will provide some guidance to help
engineers avoid the problem," Strogatz said in an interview.

Certain coincidences must occur for collective
synchronisation to occur. In the case of London's wobbly
bridge, it was large crowds walking across a flexible
footbridge that vibrated at a frequency of one cycle per
second, which just happened to be the same frequency as humans
walking.

"The people were resonating with the bridge," said
Strogatz.

As the bridge started to move, people would get in step
with the sway to steady themselves. They widened their stance
to make it more comfortable to walk and inadvertently made the
wobbling worse.

"A lot of people were blaming it on the beautiful
innovative structure, the design of the Millennium Bridge
itself, which was a radical design," said Strogatz.

"But that is not true."

Collective synchronisation occurs in nature when crickets
start chirping in unison. In some parts of the world, fireflies
blink on and off in perfect synchrony like a Christmas tree.
The monthly cycles of women living together have also been
known to synchronize.

"It is always very striking and almost spooky because it is
like order coming out of chaos," said Strogatz.

After 5 million pounds worth of modifications to steady the
structure and 20 months of closure, the Millennium Bridge
successfully reopened in February 2002.